Here's the second Key Experience Design Concept For Multitouch:
All visible content communicates how to interact with the software.
A few rules of thumb really improve usabilty:
1. Visual Cues should communicate what the underlying software is doing, how to affect it and when users have been successful.
2. All nonambient visuals communicate changes int he information structure.
3. All unnecessary elements should be eliminated to minimize the chance of confusion.
Check out the video for examples.